This invention relates generally to improvements in devices for the removal of a range of pollutant materials from flowing stormwater and the retention of these materials for later collection and disposal.
The escape of a wide range of pollutant materials into water bodies has become an important concern of environmental managers in recent years and various systems have been introduced to drainage systems for the purpose of mechanically removing litter and detritus from flowing stormwater and encouraging heavier materials to settle out so that they can be routinely removed for safer disposal.
Some systems incorporating screens have failed because their screens have become rapidly clogged by litter and detritus or because screen meshes have been made so course in an attempt to avoid clogging that they have been unable to retain many smaller items which are significant pollutants of waterways. Some systems have been unable to retain materials trapped during one storm during a later more intense storm, allowing them to be re-suspended in the more vigorous flow and thus pass downstream. This is particularly the case with captured sediments which may be completely lost if not collected frequently. Many systems are designed so as to retain putrescible materials either completely or partly immersed in water, promoting bio-degradation, causing oxygen depletion, allowing decay residues to escape downstream in solution and making handling and transport of recovered materials more difficult. Some systems retain materials in an intimate mixture of all substances captured, making their handling, transport, and sensitive disposal more difficult. Some systems depend upon or unintentionally create isolated pools of permanent water which may stagnate and provide breeding opportunities for mosquitoes.
It is an object of the present invention to provide an improved means of trapping and retaining stormwater litter and sediments which goes at least some way towards overcoming or at least minimising the prior art problems or limitation outlined above.
It is another object of the present invention to provide an improved means of trapping and retaining floating and neutrally buoyant pollutant particles as small as a few millimetres in diameter and to provide an improved means of settling and retaining heavier, fine-grained pollutant particles while generally dealing with each of the difficulties of existing trash-rack and sediment-trap systems which have been identified above.
It is a further object of the present invention to provide a stormwater litter and sediment trap which is relatively simple in both construction and operation, relatively inexpensive to manufacture and which is easy to clean and to maintain.
These and other objects of the present invention will become more apparent from the following descriptions and drawings.
According to one aspect of the invention there is provided a stormwater litter and sediment trap comprising a flume disposed so as to direct the discharge from the lower part of a pipe culvert through a gradual confining shape transition and a gradual rise in elevation to pass through an opening at the opposite end of the flume into a closely aligned or juxtaposed collection container. The trap is ideally fabricated from expanded metal mesh and especially from apertured and louvred or slatted mesh screen selected for its capacity to inhibit clogging and orientated to the flow of stormwater in a manner which maximises this characteristic and which promotes self cleansing during turbulent flows.
Preferably, beneath the flume and collection container, and extending for a distance beyond their clear edges, is a second assembly comprising a chamber, roofed with one or more layers of screen mesh of a class and orientation selected to dissipate the energy of water failing onto it so as to promote the settling of heavy particles during relatively quiescent flows and their escape trough the screen apertures into the chamber.
Discharge of water from the aforesaid chamber, preferably occurs either through a final screen wrapped in a fine-meshed filter cloth capable of retaining small particles or through a similarly filtered subsoil drainage system built into the chamber floor to allow the chamber to drain fully after pipe flow ceases. Water can also pass from the chamber through its mesh roof but does not do so in large quantities because there is a small hydraulic gradient only during high flows because the mesh induces a plane of separation because water flowing naturally downstream over the mesh surface has to sharply change direction to pass into the chamber. This means that flow through the chamber tends never to reach a velocity at which settled material is re-suspended so that little sediment is lost from the trap.
According to another aspect of the present invention there is provided a stormwater litter and sediment trap of the above described type wherein the device is installed elsewhere within the drainage system other than at an outlet e.g. further upstream.
According to a further aspect of the present invention, there is provided a method of separating litter and sediments from a flowing stream of stormwater, comprising diverting the stream of stormwater along an upwardly inclined path to a point where litter is separated and collected from the stream and diverting the sediments to a collection point below the upwardly inclined path and litter collection point.
One particular application of the present invention relates to the stormwater discharging from a pipe culvert into a watercourse, estuary or other water body and it will be convenient to describe the invention with reference to that particular application. It is to be understood that this is not to be taken to limit the scope of the invention which has application to all other forms of open and closed conduit and may be used anywhere along the line of such conduits in a covered or open situation and in single or multiple configuration.